Lagarosiphon (Lagarosiphon major)

PROHIBITED MATTER: If you see this plant report it. Call the NSW DPI Biosecurity Helpline 1800 680 244

Lagarosiphon is a perennial, aquatic plant that can dominate freshwater lakes, dams and slow-moving streams.


How does this weed affect you?

Lagarosiphon is a perennial, aquatic plant that can dominate freshwater lakes, dams and slow-moving streams. It can grow from the bottom of a water body to the surface (up to a depth of to 6.5 m in clear water or 1 m in murky water) and form dense mats several metres thick at or just below the water surface. The mats displace native vegetation and stop light penetrating the water. It is known as an ‘oxygen plant’ for aquariums however dense infestations consume more oxygen than they produce. They reduce water quality and oxygen levels and have a negative impact on native aquatic animals.

Lagarosiphon is a threat to biodiversity and has the potential to cause serious environmental damage. Infestations choke waterways and reduce the potential for recreational use (e.g. swimming, fishing, boating) and commercial use (e.g. blocking intakes in hydro-electric systems, irrigation systems and outboard motors). As with other submerged aquatic weeds, infestations are extremely difficult to control.

Where is it found?

Lagarosiphon is a native of southern Africa. It has  spread throughout the world as an aquarium plant and it is a naturalised weed in England, the Channel Islands, northern France and Italy. It is a major water weed in both the north and south islands of New Zealand. Small fragments of this plant are frequently transported on boats and trailers and infestations are often first recorded at boat ramps.

During the late 1970s, lagarosiphon was found and eradicated from a few small dams near Melbourne and Newcastle. These infestations were believed to have originated from ornamental plants in aquariums or ponds. It has also been found in a Sydney aquarium and was intercepted entering Tasmania. A cultivated specimen was recorded in Queensland in 1990. Currently there are no known infestations in New South Wales (NSW).

Distribution map

How does it spread?

Lagarosiphon spread in Australia has occurred by vegetative reproduction. Plant fragments break off and roots grow from the nodes (joints between the segments on the stem). It can move large distances downstream. It has not been known to produce male flowers, fruit or seed in Australia or outside its native range.

What does it look like?

It is important to accurately identify lagarosiphon. It can be confused with other aquatic weeds, including Elodea canadensis, Egeria densa and Hydrilla verticillata. Unlike lagarosiphon whose leaves occur in alternate spirals along the stem, these three weeds have leaves that are clustered around the stem in whorls. Lagarosiphon has a thread-like root system which branches from the stem, anchoring plants to the bottom. Rhizomes (horizontal stems) are found in the sediment and also anchor the plant.

Key identification features

  • Stems (3-5 mm in diameter and more than 5 m in length) may also be free-floating and able to reach the surface. They break easily, are sparsely branched and curved toward the base (J-shaped).
  • Leaves (5-20 mm long and 2-3 mm wide) have finely toothed margins and are more closely spaced at the top of the stem than the bottom. The tapered leaf tips curve downwards towards the stem. When the water is less alkaline, the leaves are straight.
  • Flowers are of two kinds. The very small (3 mm wide) female flower grows from a very thin white filament-like stalk. Its three petals appear clear-white or pale pink on the surface. The male flower breaks off and floats freely to the surface.

What type of environment does it grow in?

Lagarosiphon is found in high mountain streams and ponds. It grows best in clear, still or slow-moving fresh water with silty or sandy bottoms and in the cooler waters of the temperate zone (20°C–23°C with a maximum of 25°C). It prefers sheltered areas with high light intensity, and can tolerate high and low nutrient levels and alkaline pH.


Adapted by AnDi Communications from the CRC for Australian Weed Management Weed Management Guide: Lagarosiphon.

Reviewed by Rod Ensbey; Edited by Elissa van Oosterhout

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You must report this plant if you see it anywhere in NSW. Call the helpline listed above. Help will then be provided to remove and destroy it. This serious weed could spread if control efforts do not follow all protocols. Not reporting it is a breach of your legal biosecurity duty.

Herbicide options

Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website

See Using herbicides for more information.

Diquat 200 g/L (Reglone®)
Rate: 5 L per megalitre water
Comments: Apply by stem injection below the surface or as a surface spray
Withholding period: 1 day in pasture, 10 days in treated water.
Herbicide group: L, Inhibitors of photosynthesis at photosystem I (PSI inhibitors)
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area Duty
All of NSW General Biosecurity Duty
All plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
All of NSW Prohibited Matter
A person who deals with prohibited matter or a carrier of prohibited matter is guilty of an offence. A person who becomes aware of or suspects the presence of prohibited matter must immediately notify the Department of Primary Industries

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For technical advice and assistance with identification please contact your local council weeds officer.
For further information call the NSW DPI Biosecurity Helpline on 1800 680 244 or send an email to

Reviewed 2018